A substrate processing system, which performs a predetermined processing, for example, a film forming processing using plasma on a semiconductor wafer (hereinafter, simply referred to as a “wafer”) as a substrate, includes a plurality of processing chambers and a conveyance chamber connected to each processing chamber, and carries a wafer into or from each processing chamber by a conveyance arm disposed in the conveyance chamber. Each processing chamber includes a placing table, and each wafer is placed on the placing table. However, in order to accurately perform a desired processing on the wafer, it is necessary to accurately place the wafer at a desired position on the placing table. Thus, the position of the wafer is measured by a sensor during the conveyance of the wafer by the conveyance arm, a displacement of the wafer from the desired position (hereinafter, referred to as a “positional displacement”) is calculated, and a trajectory of the conveyance by the transport arm is adjusted so as to correct the positional displacement. In the following descriptions, a portion moving integrally between a node (bendable point) and another node in the transfer arm is referred to as a “link,” and the tip portion (terminal portion) of the conveyance arm is referred to as an “end effector.” Further, a two-link type conveyance arm refers to a conveyance arm having two links, and a one-link type conveyance arm refers to a conveyance arm having only one link.
In a conventional substrate processing system, a two-link scalar type or frog-leg type conveyance arm has been used as a conveyance arm. In the scalar type or frog-leg type conveyance arm, a bifurcated fork holding a wafer moves rectilinearly without rotating. For example, in a scalar type conveyance arm 90 illustrated in FIGS. 9A and 9B, two arm portions 93, 94, which connect a pedestal 91 and a fork 92, rotate. However, when the wafer W is conveyed as the arm portions are shifted from a contracting state (FIG. 9A) to the expanding state (FIG. 9B), the fork 92 and the wafer W moves rectilinearly, but the fork 92 and the wafer W do not rotate.
The substrate processing system specifies a position of an outer periphery (edge) of the wafer W from a positional relationship between a position of a sensor 95 disposed on a conveyance trajectory of the wafer W and a position of the center of the fork 92 of the conveyance arm 90 (a cross-shaped intersecting point indicated by the broken lines in FIGS. 9A and 9B) when the edge of the wafer W crosses a facing surface of the sensor 95 (covers the sensor 95). In general, two sensors 95 are provided, and the edge of the wafer W crosses the facing surfaces of each of the sensors 95 twice. Thus, four positions of the edge of the wafer W are specified. Then, a position of the center of the wafer W is calculated from the four specified positions of the edge to calculate the positional displacement of the wafer W (see, e.g., Japanese Patent Laid-Open Publication No. 2013-042112). At this time, the position of the center of the fork 92 and the position of the sensor 95 in a coordinate system based on the rectilinear direction of the wafer W (hereinafter, referred to as a “rectilinear direction coordinate system”) may be specified from an encoder value of the conveyance arm 90. Meanwhile, in order to calculate the position of the center of the wafer W, each of the positions of the edge of the wafer W needs to be specified (plotted) in a coordinate system 96 based on the center of the fork 92 holding the wafer W (hereinafter, referred to as a “fork coordinate system”). However, as described above, since the fork 92 and the wafer W move rectilinearly without rotating, the rectilinear direction coordinate system coincides with the fork coordinate system 96 during the conveyance of the wafer W. Thus, each of the position of the edge of the wafer W specified in the rectilinear direction coordinate system may be regarded as each position of the edge of the wafer W in the fork coordinate system 96.
Incidentally, as the processing temperature of the wafer W has recently been increased, the wafer W may be heated to, for example, 700° C. to 800° C. in the processing chamber. In this case, the fork 92 and the arm portions 94 of the conveyance arm 90 are also heated by heat transfer from the heated wafer W. Thus, a bearing or a drive belt, which is embedded in the fork 92 or the arm portions 94, may be damaged by heat. Therefore, a one-link type conveyance arm has been suggested, which includes an end effector in which the fork 92 and the arm portions 94 are formed integrally in order to eliminate the bearing and the drive belt, thereby improving the heat resistance. In the one-link type conveyance arm, the end effector holds the wafer W when the wafer W is conveyed.